1. Field of the Invention
The present invention relates to an automatic exposure apparatus for printing plates and a method for exposing printing plates, in which a printing plate is disposed at a predetermined position on a top surface of a surface plate so as to record an image on the printing plate.
2. Description of the Related Art
In a conventional automatic exposure apparatus for printing plates, for example, printing plates in which photosensitive layers (e.g., photopolymer layers) are provided on supports are used, and the printing plates are accommodated in an accommodating cassette. Further, a table is provided in the interior of the automatic exposure apparatus for printing plates, and the printing plate conveyed from the accommodating cassette is disposed at a predetermined position on a top surface of the table. A plurality of through-holes and a plurality of sucking grooves communicating with the through-holes are formed on the top surface of the table. When air is sucked from the through-holes by a vacuum pump, the interior of the through-holes and the sucking grooves is brought to a vacuum state. As a result, the printing plate disposed on the top surface of the table is in a state of being vacuum-sucked onto the top surface of the table. In this state, an image is directly recorded on the photosensitive layer of the printing plate by laser beams or the like.
However, in such automatic exposure apparatus for printing plates, when the printing plate is singly stored, the printing plate may curl naturally. If this curled printing plate is disposed on the top surface of the table, the printing plate is raised up further from off the top surface of the table towards the outer peripheral portion of the printing plate. Accordingly, even when an attempt is made to suck this curled printing plate onto the top surface of the table, sometimes the printing plate cannot be sucked so as to be in complete contact with the top surface of the table with only the normal sucking ability of the vacuum pump (i.e., the printing plate partially comes up from off the top surface of the table). As a result, such a problem may be caused that a defective image such as an unfocused image is formed when an image is recorded onto the printing plate. This problem is particularly remarkable when the support is made of aluminum so as to form a hard printing plate (so-called photopolymer plate).
As a related art of the present invention, a device for sucking sensitive materials is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-268524. This device consists of a table in which many suction holes and grooves are formed, essentially three air hoses which communicate with the suction holes, three solenoid valves each of which is provided at each of the air hoses, and a vacuum pump. Initially, one of the solenoid valves is opened, and a sensitive material is sucked with a predetermined suction force by the many suction holes communicating with the vacuum pump. Then, the other two solenoid valves are opened, and the same sensitive material is sucked with a suction force which is weaker than the former predetermined suction force. However, in this device, the suction force for the sensitive material depends solely upon sucking ability of the vacuum pump, and there is no structure for increasing the suction force by an element other than the vacuum pump. Accordingly, the sensitive material is not sufficiently sucked, and this may result in a drawback such as a defective image being formed.
In consideration of the above facts, it is an object of the present invention to obtain an automatic exposure apparatus for printing plates and a method for exposing printing plates, which can prevent a defective image such as an unfocused image from being formed when an image is recorded onto the printing plate.
In accordance with a first aspect of the present invention, a sucking-holding device for sucking and holding a flexible thin plate onto a top surface of a base comprises: a suction path, which has a plurality of apertures on the top surface of the base; a suction unit, which is for sucking air from the apertures via the suction path; a suction line, which communicates with the suction path and the suction unit; a valve device, which is provided on the suction line, and which can open and close the suction line; a hollow body, which is provided on the suction line between the valve device and the suction unit, and which has a predetermined internal capacity whose pressure can be decreased by an operation of the suction unit; and a controller, which can open the valve device, when the internal pressure of the hollow body has reached a predetermined value or lower, or in accordance with a duration of operation of the suction unit. In this case, when the suction line is opened by an operation of the valve device, the hollow body whose internal pressure has been decreased and the suction path (thus, the apertures) communicate with each other, and the flexible thin plate is sucked and held onto the top surface of the base. At this time, a strong suction force is temporarily applied to the flexible thin plate via the apertures, because of the existence of the hollow body whose internal pressure has been decreased in the suction line. As a result, the flexible thin plate can be reliably fixed instantaneously on the top surface of the base.
In accordance with a second aspect of the present invention, a sucking method, in a sucking device for sucking a flexible thin plate onto a top surface of a base, the device comprising: a suction path, which has a plurality of apertures on the top surface of the base; a suction unit, which is for sucking air from the apertures via the suction path; a suction line, which communicates with the suction path and the suction unit; a valve device, which is provided on the suction line, and which can open and close the suction line; and a hollow body, which is provided on the suction line between the valve device and the suction unit, and which has a predetermined internal capacity whose pressure can be decreased by an operation of the suction unit; comprises steps of: (a) operating the suction unit so as to decrease the internal pressure of the hollow body; and (b) opening the suction line via the valve device. In this case, the internal pressure of the hollow body is considerably decreased by an operation of the suction unit. Then, the interior of the hollow body and the suction path communicate with each other by an operation of the valve device, and a strong suction force is generated instantaneously at the apertures. As a result, the flexible thin plate is sucked instantaneously, and fixed onto the top surface of the base.
In accordance with a third aspect of the present invention, an exposure apparatus, which includes a sucking device for sucking a printing plate onto a top surface of an exposing base, and which records an image onto the printing plate, comprises: a suction path, which has a plurality of apertures on the top surface of the exposing base; a suction unit, which is for sucking air from the apertures via the suction path; a suction line, which communicates with the suction path and the suction unit; a valve device, which is provided on the suction line, and which can open and close the suction line; a hollow body, which is provided on the suction line between the valve device and the suction unit, and which has a predetermined internal capacity whose pressure can be decreased by an operation of the suction unit; and a controller, which can open the valve device when the internal pressure of the hollow body has reached a predetermined value or lower. In this case, the printing plate, which is generally formed of a plurality of layers and curls easily, is sucked instantaneously due to a low pressure of the interior of the hollow body which has a large capacity. As a result, the printing plate can be in contact with and fixed onto the top surface of the exposing base. Therefore, the formation of defective images is decreased or eliminated in the exposure process which follows.